The present invention relates to a two-ply screen (fabric) for the sheet forming zone of a papermaking machine consisting of weft filaments arranged in pairs one over the other and of warp filaments, all the warp filaments being woven into the top layer of the screen.
Such screens have been known from German OS No. 2,263,476 and OS No. 2,540,490. In these screens all the warp filaments on the backing side pass below the weft filaments to that they are exposed to abrasion. Two-ply screens break immediately when the warp filaments are worn through since the warp filaments transmit all the driving force exerted on the screen. Thus, wear of the warp filaments is the normal cause of screen failure. This primarily applies to the screens described in German OS No. 2,263,476 in which the warp filaments are worn through long before the weft filaments are consumed on the backing. German OS No. 2,540,490 proposes to overcome this difficulty and partially solves this problem in that on the backing the warp filaments pass under only one weft filament. Consequently the warp bends on the backing are substantially shorter and the weft bends are longer. Therefore, the crimp of the weft filaments can be enlarged so that the warp filaments on the backing are quasi embedded in the weft filaments. By means of sufficient setting one can make sure that the weft filaments are worn through before the warp filaments are. However, it is a disadvantage that the weft filaments may be only a few hundredths of a millimeter thicker than the warp filaments because otherwise the warp filaments are urged downwardly and outwardly by the thicker and stiffer weft filaments and thus more exposed to wear. Therefor, in such a screen it is not possible to use thicker weft filaments to provide more volume for abrasion.
Numerous attempts have been made to prolong the service life of multi-ply papermachine screens. For example, German OS No. 2,455,185 teaches assembling a papermachine screen from two separate fabric webs interconnected by a special binder warp. The binder warp, however, extends partially on the backing beneath the weft filaments so that it is exposed to wear. The thinner the binder warp the sooner will it be worn through. Such a screen composed of two separate fabric webs is very expensive and complicated. Also, European patent application No. 0 010 311 teaches the use of a highly reduced number of weft filaments on the backing of a two-ply papermachine screen to make the screen more permeable. In order to prolong the service life a number of warp filaments are woven only into the lower layer of the screen to thereby increase the volume available for abrasion. These warp filaments in the bottom layer also determine the spacing of the warp filaments in the top layer. The small number of warp filaments on the papermaking side results in substantial screen marks in the paper. It is a special disadvantage of this screen that there is a small number of weft filaments and a large number of warp filaments on the backing side. Both features have the consequence that in each repeat three or four warp filaments wrap around each weft filament so that the latter have no free length to bend outwardly toward the backing side. This screen is thus a typical warp runner and breaks after a relatively short time, namely as soon as the exposed bends of the warp filaments are worn through. Although FIG. 5A of the European patent application shows a screen in which a number of warp filaments do not bend downwardly, i.e. they are not woven into the bottom layer, the filament ratio on the backing is nevertheless unfavorable because more warp filaments than on the papermaking side are woven into the few weft filaments on the backing side.